IE74164B1 - Alloy for glass fibre centrifuges - Google Patents

Abstract

Nickel-based alloy forming part of a centrifuge for glass fibres. Its composition contains, in particular, the elements chromium, tungsten and carbon, the proportions of which are chosen so that the alloy exhibits carbides of the M23C6 type in its crystalline structure, M being chromium and/or at least one "equivalent" metal, the carbides M23C6 being essentially secondary.

Description

ALLOY FOR GLASS FIBRE CENTRIFUGES The present invention relates to an alloy intended to be used in the manufacture of centrifuges. This alloy is particularly suitable for constructing centrifuges which produce glass fibres.

A centrifuge of this type rotates about its vertical axis at a speed of the order of 800 to 4000 rpm. A large number of apertures are provided on its peripheral wall. An annular combustion chamber adjacent the centrifuge produces a descending gaseous current which passes along the peripheral wall of the centrifuge and draws the thin streams of glass emerging from the. apertures in order to produce a small glass fibre.

Molten glass is introduced into the centrifuge and is projected under the action of 2θ centrifugal force towards the inner face of the peripheral wall of the centrifuge. The molten glass passes through the apertures in this wall. The thin glass streams produced outside the centrifuge are then drawn by the action of the gaseous current.

The stresses to which the centrifuge is subject are threefold: thermal shock on stopping and starting, hot flow owing to the centrifugal forces, and corrosion of the fibre drawing apertures by the glass. As an example, the operating temperatures are of the order of 10001200°C. These are the temperatures at which the glass is at a suitable viscosity.

In view of the particularly harsh operating conditions, centrifuges sooner or later deteriorate when used. These centrifuges may have to be replaced for various reasons, such as: deformation of the fibre-drawing strip, the appearance of horizontal or vertical cracks, large scale wear of the apertures.

In practice, the most frequent reasons for changing centrifuges are still deformation of the peripheral strip which is manifested by a deterioration in the quality of the fibres produced. This deformation is connected both with the effects of centrifuging on the peripheral strip and weakening of the latter by erosion of the apertures. Nevertheless, an improvement in resistance to corrosion and to deformation cannot be separated from questions concerning the fragile nature of the alloy. It is particularly important to avoid the centrifuges rupturing during operation. It is therefore necessary to select the centrifuge alloy such that it has good resistance to deformation, on the one hand, and, on the other hand, such that it is not too fragile. Since these two features are at least in part contradictory, a compromise solution must be reached.

Ni and Cr based alloys for constructing centrifuges of this type are known, in particular from patent FR 2 459 783. This patent recommends an alloy composition which is relatively precise as regards the principal constituents, in particular the carbon content.

Centrifuges produced from the alloy of the prior art on the whole display improved properties. Overall these centrifuges have satisfactory useful lives. Nevertheless, some centrifuges among those complying with the features of this document have useful lives which are far shorter than the average useful life without any possible explanation and thus any means of foreseeing the reasons for these incidents .

In view of this situation, the inventors sought a solution enabling centrifuges having the desired qualities of a long useful life to be produced systematically. In other words, the inventors endeavoured to eliminate all risks of centrifuges being rendered obsolete prematurely.

The inventors achieved this result by establishing the particular features relating to the alloy structure and in particular the nature and morphology of the carbides in this alloy. The inventors also determined the features enabling this result to be achieved and which concern the alloy composition and metallurgical processes used during the manufacture of the centrifuge.

On the basis of Ni-Cr alloys of the type mentioned in the cited prior art, the invention proposes a composition comprising the following given as percentages by weight: Cr 27.5 - 29.5% W 6.5 - 7.8% C 0.69 - 0.73% Fe 7 10% and, optionally, Si and Mn, each especially in a percentage by weight of 0.6 to 0.9%. The alloy also comprises in its structure substantially secondary M23C6 type carbides dispersed finely and homogeneously in the structure, which guarantees that the properties of the centrifuges produced are the best possible. M is chromium and/or at least one equivalent metal contained in the alloy, the content of chromium equivalent not exceeding 38%.

The invention relates also to the centrifuge for glass fibres, which is constituted by an alloy of this type and is obtained by casting and heat treatment of that alloy.

The inventors demonstrated by metallurgical studies, which will be dealt with in further detail with respect to the examples, that the qualities of the centrifuges and especially their resistance to plastic flow were linked to the presence of carbides having well-defined natures and morphologies. Having identified these structural elements of the centrifuge alloy, the inventors were also able to establish the conditions which determine, or at least promote, the formation of these structural elements.

Thus, in mechanisms on which the control of the plastic flow of the material of which the centrifuge is comprised depends, the inventors firstly noted the essential role of carbides.

These carbides would come into play owing to their aptitude to block dislocations present in the metal network. Dislocations, however, are onlyblocked efficiently if the carbides in question are in a sufficiently fine and well dispersed form.

On the basis of alloys used previously for their aptitude to meet the various requirements, in particular mechanical and chemical resistance at the temperatures in question, the inventors have shown that, in order to obtain good resistance to plastic flow, the carbides should be present in the alloy in a proportion which is as high as possible in the M23C6 form, M being Cr or an equivalent metal (W, Si, Zr) from the point of view of the structure.

The inventors' studies have shown the complexity of the mechanisms leading to these particular carbides. They are due to the diverse nature of the phases which appear during solidification of the molten alloy and during any subsequent heat treatments .

In practice, these centrifuges are produced by casting the molten alloy. The cast centrifuge is conventionally the subject of a heat treatment in order to rearrange the structure, which arrangement cannot be attained directly owing to solidification during casting.

According to the invention, the heat treatment to which the alloy is subjected affects principally the appearance and distribution of the carbides in the matrix.

In the following, primary carbides and secondary carbides will be used respectively to designate the carbides which appear after casting on the one hand and after heat treatment on the other.

The invention proposes an alloy composition comprising specific proportions of chromium and tungsten such that the final crystalline structure of the alloy comprises M23C5 type carbides, which, for reasons to be given subsequently, are substantially secondary and distributed homogeneously in the matrix.

In accordance with the invention, the chromium equivalent content is preferably not more than 38% and is advantageously at most 37% in order to promote the formation of the M23C6 secondary carbides.

In addition, in order to maintain good resistance to corrosion at high temperature, in accordance with the invention, the chromium equivalent content is preferably not less than 35% and advantageously not less than 35.5%.

According to the invention, in order to improve resistance to corrosion at high temperature and resistance to plastic flow, the chromium content is between 27.5 and 29.5% and preferably between 27.5 and 28.5%.

Tungsten contributes to the hardness of the alloy and resistance to plastic flow. Its content is between 6.5 and 7.8% and preferably between 7.2 and 7.6 % .

Carbon is an essential element for the formation of carbides. Its content is between 0.69 and 0.73%.

A small proportion of iron is used in the alloy composition. It improves resistance to sulphuration, as nickel-based alloys are highly sensitive to sulphur present in the glasses used. Its content is advantageously between 7 and 10%.

Further elements are also used if necessary in the alloy composition, either in the form of traces introduced with the principal elements or as complementary elements to perfect given properties .

Thus, in accordance with the invention, a small amount of silicon, 0.6 to 0.9%, increases the alloy's hardness and resistance to plastic flow.

A preferred alloy composition according invention has the following Ni 54.5 - 58% Cr 27.5 - 28.5% W 7.2 7.6% C 0.69 0.73% Si 0.6 0.9% Mn 0.6 0.9% Fe 7 - 10% According to the invention, a suitable heat treatment enables the secondary carbides to be distributed homogeneously in the metal matrix, thereby hindering the spread of dislocations in the matrix. The inventors have shown that for good resistance to plastic flow it is advantageous to have a large number of very fine secondary carbides distributed homogeneously.

Accordingly, the inventors have noted that it is preferable if the temperature of the crude solidification structure of the alloy increases rapidly. The inventors have noted that a slow increase in temperature leads to a nucleation stage of long duration. The first nuclei created grow and can coalesce at the same time as the final nuclei appear. A more rapid increase in temperature enables the simultaneous existence of nucleation mechanisms and coalescence of secondary carbides to be avoided. Subsequently, maintaining the alloy at a relatively high temperature for a given amount of time principally encourages the growth of these carbides. A high number of nuclei enables the formation of carbides which are too bulky to be avoided.

It has proved desirable to optimise this process. It is thus preferable to operate at a level temperature which is not too high so as to avoid the carbides coalescing. Moreover, a less high level temperature corresponds to a longer treatment time. A compromise therefore has to be reached.

In this respect, it has appeared advantageous to select a temperature increase rate which is not less than 3°C/mn.

Further in this respect, according to the invention, it is preferable to maintain the temperature of the heat treatment at less than 1000°C and preferably less than 900°C. 1θ The duration of the heat treatment in these conditions is at least 5 hours and preferably at least 8 hours.

An advanced metallurgical study enabled the inventors to discover the influence of the alloy composition, in particular the chromium equivalent content and, more precisely, the chromium content, on the nature and morphology of primary carbides on the one hand (before heat υ treatment) and secondary carbides on the other hand (after heat treatment).

By means of an image analysis technique, the inventors discovered that after the heat treatment alloys having a composition similar to that in the patent FR 2 459 783 have secondary carbides of the M23C6 type. However, the nature of these carbides differs from one alloy to another.

Some alloys have fine M23C6 carbides. Other alloys have fine M23C6 carbides and M23C6 carbides with the appearance of Chinese script. Sometimes only M23C6 carbides with the appearance of Chinese script were noted.

A comparison with the specific composition of each of these alloys enabled the inventors to discover that the alloys having a chromium equivalent percentage of more than 38% comprise either M23C6 type carbides resulting from the heat treatment (secondary carbides), or carbides with the appearance of Chinese script, or fine carbides.

Alloys with a chromium equivalent composition of the order of 38% consist of carbides resulting from the heat treatment (secondary carbides) of the substantially fine M23C6 type.

The chromium equivalent content thus has a direct influence on the nature of the secondary carbides .

Furthermore the inventors analysed the nature of primary carbides (before heat treatment) according to the alloy composition and especially according to the chromium equivalent content.

They observed that alloys having a similar composition to that of patent FR 2 459 783, of which the chromium equivalent content is greater than 38%, have both primary carbides of the M23C6 type with the appearance of Chinese script and spearhead M7C3 type carbides. Alloys of which the chromium equivalent content is of the order of 38% have primary carbides of the M7C3 type with a spearhead morphology.

By combining these two observations, firstly the nature of primary carbides and secondly the nature of secondary carbides as a function of the chromium equivalent content, the inventors reached the conclusion that the nature of primary carbides (before heat treatment) determines the nature of secondary carbides (after heat treatment), ie: . M7C3 primary carbides of the spearhead type are converted during heat treatment (10 hours at 850°C) into fine secondary carbides of the M23C5 type; . primary carbides of the M23C6 type with the appearance of Chinese script are not subject to any modification as regards their nature or morphology during heat treatment irrespective of its type.

The nature of primary carbides is itself directly connected with the chromium equivalent content of the alloy.

The chromium equivalent content thus determines indirectly the nature of the secondary carbides by means of the nature of the primary carbides .

The critical chromium equivalent content is 38%. Beyond this proportion M23C6 type primary carbides with the appearance of Chinese script form in addition to the M7C3 spearhead type primary carbides .

A more detailed description of the tests as well as an explanation of the crystalline modifications of the alloy studied during cooling are described hereinbelow with reference to the drawings in which: . Figure 1 shows a section through a pseudo-binary Cr-Ni-C diagram with 0.7% carbon; . Figure 2 illustrates example 1; the plastic flow curves of various samples differing by the nature of the solidification carbides (primary carbides) are shown therein; . Figure 3 illustrates example 5; this is an illustration of the plastic flow curves of samples subjected to different heat treatments.

The applicants prepared a pseudo-binary Ni-Cr-C diagram (Figure 1) with 0.7% carbon from 90 experimental smeltings. The basic composition of the alloy is that used in patent FR 2 459 783 with the exception of the nickel and chromium content. The TSD technique was used for this purpose.

This technique consists in placing a bar of the alloy in question in an aluminium oxide tube. A field magnet enables a 4-5 cm area to be melted locally. A thermo-element is placed at this point to check the temperature. When the area has melted, the assembly comprising the aluminum oxide tube and the sample is drawn at a constant speed. As it emerges from the area of activity of the field magnet, the liquid starts to solidify at a known rate and independently of the drawing speed. When a sufficient length has solidified unidirectionally, the assembly is cooled at 70°C/s in a water box. Areas which were liquid before this violent cooling process are solidified with a very fine structure which easily distinguishes them from the parts which were already solid before the abrupt cooling process.

The Cr-Ni-C pseudo-binary diagram is prepared on the basis of a fixed composition where only the chromium and nickel contents vary. The contents of the other elements in percentages by weight are: carbon tungsten silicon manganese 0.7% 7.2 - 7.6% 0.6 - 0.9% 0.6 - 0.9% the remainder principally being iron.

This diagram enables a qualitative indication to be provided as regards the nature ofthe phases present in the alloy as a function of the content of chromium and nickel and of the alloy temperature.

The existence of a maximum relative to the area: liquid + % phase (austenitic matrix) + M7C3 type carbides - (K2 + L + % ) is established for a chromium content of the order of 28%, M7C3 = Cr6(Fe,Ni)0 8 W0 2 C3 in spearhead configuration.

Using this phase diagram, the development of the nature of the phases present in a composition in the liquid phase during its solidification can be followed.

For example, the remarkable composition of the order of 28% chromium can be selected and the development of the phases present when the liquid is cooled until it solidifies may be followed.

The following conversions are noted ( Figure 1): . at point 1, some of the liquid is converted into the austenitic phase: L —L + % (phases present : L +^).

If cooling is continued, the remainder of the liquid will be converted at point 2 into phase K2, ie. M7C3 = Cr6( Fe ,Ni) 0>8 WQ<2 C3: L —K2 (phases present : K2 + Y ) .

If the chromium content is now set at more than approximately 28%, the corresponding liquid phase undergoes the following conversions (Figure 1) during cooling: . at point a, as previously, some of the liquid phase is converted into the austenitic phase ( % ): L —(phases present : L + ); . at point b, some of the remaining liquid phase is converted into the K2 phase (M7C3): L —K2 (phases present : L + + K2); . there is an additional conversion at point c, relative to the ideal content of 28% chromium which corresponds to a peritectic conversion. The remainder of the liquid phase is converted into phases K2 and K3 on either side of point c: L —K2 + K3 (phases present : / + K2 + K3).

K3 is of the M23C6 type where M23C5 = Cr17(Fe,Ni)5 Wj C6 with the appearance of Chinese script.

Depending on the chromium content in the alloy, the M23C6 phase may thus occur therein.

If the critical chromium content of approximately 28% is exceeded, the crude solidification structure presents M7C3 type and M23C6 type primary carbides.

For a chromium content of 28%, the crude solidification structure has primary carbides substantially of the M7C3 type (Crg(Fe,Ni )0 θ WQ 2 C3) .

New phases appear for chromium contents lower than this value.

The nature of the phases was identified by image analysis.

According to this diagram, the chromium content of the alloy in question plays an essential role with respect to the nature and morphology of primary carbides.

This result agrees perfectly with the preceding results, ie. the influence of the chromium equivalent content on the nature of primary carbides. So-called chromium equivalent metals principally include chromium and metals referred to as equivalent to chromium. The influence of these so-called equivalent metals, such as tungsten, on the nature of primary carbides present in the alloy is considerable.

Thus the inventors have demonstrated the occasional presence of some areas which are richer in chromium or tungsten within the structure of the alloy in question. Segregations then appear which cause M23C6 type primary carbides to be precipitated. The inventors explain this irregularity in chromium or tungsten by an enrichment with these elements during solidification of the residual liquid. Thus the chromium content is preferably restricted to 28.5% in order to be certain not to precipitate primary M23C6 carbides .

The subsequent heat treatment does not alter the morphology of primary carbides. The preparation conditions, such as the speed of solidification determined, for example, by the casting temperature, may have an effect on the nature and morphology of primary carbides in addition to the alloy composition.

The inventors have performed various tests in order to correlate their observations regarding the nature and morphology of primary and secondary carbides with the alloy's resistance to plastic flow, the tests being indicative of the useful life of centrifuges.

Thus the inventors found that the nature of secondary carbides has a very clear influence on the alloy's resistance to plastic flow.

The presence of fine grained secondary carbides resulting from M7C3 spearhead type primary carbides imparts good resistance to plastic flow to the alloy. On the other hand, the presence of secondary carbides with the appearance of Chinese script, which are more bulky, imparts poor resistance to plastic flow to the alloy.

One possible explanation for this observation is that plastic flow is due to a propagation of dislocations through the alloy.

Fine secondary carbides are more efficient in preventing these dislocations from spreading than more bulky carbides. Fine carbides, which are more numerous, may easily be dispersed throughout the entire alloy.

More bulky carbides with the appearance of Chinese script do not impede the spreading of dislocations so easily. The dislocations may easily bypass any obstacles and continue to spread.

Fine secondary carbides distributed homogeneously in the alloy matrix will thus provide better resistance to plastic flow than more bulky carbides distributed in an irregular manner.

In view of these results, the inventors then sought to optimise the heat treatment to which the alloys are subjected. The primary carbides are converted into secondary carbides during the heat treatment.

The inventors have demonstrated the influence, firstly, of the rate of the increase in temperature on the nucleation of secondary carbides and, secondly, the influence of the time and level temperature on the growth of these secondary carbides .

A relatively high temperature increase rate avoids the simultaneous presence of nucleation phases and coalescence of secondary carbides. The growth of the latter is facilitated by a lower level temperature. On the other hand, in order to obtain carbides of a suitable size, the time for which they are maintained at this temperature is all the more long, the lower the level temperature. A compromise has to be found between these two parameters.

Example 1 illustrates the influence of the alloy composition on resistance to plastic flow. Examples 2 to 5 illustrate the influence of time and temperature parameters on the distribution of carbides throughout the matrix.

EXAMPLE 1 Three series of samples were prepared in order to test their resistance to plastic flow.

The samples are subjected to a tensile force of 35 MPa at 1000°C and their deformation is measured as a function of time. These three series have respectively three different chromium equivalent contents. They were subjected to a heat treatment for 10 hours at 850°C.

Figure 2 shows the correlation between the chromium equivalent content and the resistance to plastic flow of the alloy.

The X axis shows the time in hours; the Y axis shows the deformation of the sample as a percentage. This plastic flow curve (timedeformation) is a practical means of quantifying the useful life test, of a centrifuge consisting of an alloy of a given composition.

For each series of samples, a band is obtained which is delimited by two curves corresponding to the resistance to plastic flow of samples having a given composition.

The bands delimited by curves 1, 2 and 3 correspond respectively to a chromium equivalent content of: 38%, 39.2% and 41.1%.

: Composition : Nature of : Deformation : Time of : primary on rupture ‘ rupture : Cr : W : Cr.e : carbides (%) (h) : (%) : (%) : (%) : ----------;-----; — ------ : — . --------- : 29 : 8,7 : 38 : 100 % M-tCj : 3-4 : 300 : : 30,7 : 7,6 : 39,2 : 50 % M23Ce : 1 O in M,CS : 4 : 60 to 130 : : 31 : 9,1 : 41,1 : 100 % M„Ce : 3 : <30 : It should be remembered that Creq = Cr + w + Si + Nb.

The nature of the carbides and their distribution were identified by image analysis.

The best results correspond to a minimum deformation for a maximum amount of time.

According to these results, it appears that a chromium equivalent content of more than 38% has a harmful effect on the resistance to plastic flow of an alloy of this type.

EXAMPLE 2 Two series of samples having the following composition were prepared: carbon 0.69 - 0.73% tungsten 7.2 - 7.6% chromium 28.5 - 29.5% nickel 54.5 - 58% the remainder essentially comprising iron.

Each series of samples was subjected to heat simulation on a dilatometer.

The temperature increase rate is varied respectively from 6°C/mn and l°C/mn to a temperature of 1000°C. At this temperature, the samples are cooled abruptly. The carbides are then analysed by image analysis.

The inventors have demonstrated the nucleation of secondary carbides during this increase in temperature. The rate of 6°C/mn promotes nucleation which is more homogeneous.

EXAMPLE 3 Various samples were prepared according to the composition in Example 2.

They were subjected to the following heat treatments : Time sample maintained temperature at Temperature at which sample maintained • Results 10 h 850°C : fine homogeneous ; secondary carbides 4 h < t < 10 h 950eC : beginning of coalescence 4 h < t < 10 h 1050®C • beginning of coalescence 4 h 1190*C • strong coalescence Coalescence of the carbides impairs resistance to plastic flow; the carbides are no longer dispersed homogeneously in the matrix.

They form masses here and there, which can easily be evaded by dislocations.

Beyond 850°C, the higher the temperature, the greater the coalescence of the secondary carbides. Maintaining them at a lower temperature provides better results .

EXAMPLE 4 The influence of the time for which the samples are maintained at a given temperature was studied. The temperature at which they are maintained is 1000°C.

The composition of the samples is the same as in Example 2.

Time Results growth of secondary carbides progressive coalescence — elongate carbides A time for which the samples are maintained at a given temperature of less than 2 hours is more beneficial to good resistance to plastic flow than a time greater than this value for a temperature of 1000°C. The more this time is increased, the more the secondary carbides coalesce and no longer obstruct the spread of dislocations, which is the cause of certain plastic flow.

Thus according to Examples 3 and 4 a compromise is to be found between the time and level temperature during the heat treatment.

EXAMPLE 5 Various samples were prepared from the composition of Example 2 and subjected to plastic flow tests. Figure 3 shows the behaviour with respect to plastic flow of two series of samples subjected to different heat treatments. Curve 1 corresponds to a heat treatment of 10 hours at 850°C; a lesser degree of deformation is obtained with respect to the samples which were subjected to a heat treatment for 4 hours at 1000°C (curve 2). A lower level temperature provides better results. The band corresponding to the treatment of 10 hours at 850°C is shifted towards the greater times for lesser degrees of deformation.

Treatment Deformation Speed (area siope II x 10 ) h at 1000’C 1.9 to 2.3 h at 850’C 0.38 too.05 The influence of the level temperature on 10 resistance to plastic flow is greater than the influence of the duration for which the sample is maintained at this temperature (Examples 3 and 4). It is preferable to operate at a lower temperature for a longer amount of time.

These results emphasise the efficiency of the optimised heat treatment.

Claims (5)

1. Nickel-based alloy used in the manufacture of a glass fibre centrifuge and of 5 which the composition comprises the following elements given as percentages by weight: Cr 27.5 - 29.5% W 6.5 - 7.8% C 0.69 - 0.73% Fe 7 10% and, optionally, Si and Mn, each especially in a percentage by weight of 0.6 to 0.9%, the remainder substantially being nickel and having in its crystalline structure fine, substantially secondary M 23 C 6 type carbides distributed homogeneously in the alloy, M being chromium and/or at least one equivalent metal, contained in the alloy, the content of chromium equivalent not exceeding 38%.

2. Alloy according to Claim 1, of which the composition comprises a chromium equivalent content (chromium and/or at least one equivalent metal) of between 35 and 38%.

3. Alloy according to Claim 2, of which the composition comprises a chromium equivalent content of between 35.5 and 37%. 5

4. Alloy according to any one of the preceding claims, of which the composition is substantially: Ni 54.5 - 58% Cr 27.5 - 28.5% W 7.2 - 7.6% C 0.69 - 0.73% Si 0.6 - 0.9% Mn 0.6 - 0.9% Fe 7 10% 5. Glass fibre centrifuge, characterised in that it is constituted by the alloy according to any one of Claims 1 to 4, obtained by casting and heat treatment. 6. Centrifuge according to Claim 5, characterised in that after it has been formed by casting the alloy, it is subjected to a heat treatment comprising an increase in temperature to a level, a level temperature and a period of maintenance at this level, selected in such a manner that the primary carbides resulting from the casting process are substantially converted into secondary carbides, which are not very bulky and are distributed homogeneously. 5 7. Centrifuge according to Claim 6, wherein the rate of temperature increase is selected so as to be sufficiently rapid to avoid nucleation phases and coalescence of the carbides being present simultaneously. 8. Centrifuge according to Claim 6 or 7, wherein the temperature of the heat treatment is increased at a rate of at least 3°C/mn. 15 9. Centrifuge according to any one of Claims 6 to 8, characterised in that the level temperature of the heat treatment is not greater than 1000°C and preferably less than 900°C. 20 10. Centrifuge according to any one of Claims 6 to 9, wherein the heat treatment is maintained at the level temperature for at least 5 hours. 11. Centrifuge according to any one of Claims 25 5 to 10, wherein the duration of the heat treatment is at least 8 hours. 12. Nickel-based alloy used in the manufacture of a glass fibre centrifuge according to Claim 1, substantially as herein described in the Examples.

5. 13. Glass fibre centrifuge according to Claim 5, substantially as herein described.